Question

Why might a `"KBr"` pellet be used in IR spectroscopy?

Answer 1

Consider the main point of IR.

What you're doing in IR spectroscopy is disturbing the motion of molecules with resonant frequencies to make them stretch and bend more intensely. Do ionic bonds stretch and bend?

FREQUENCY VS FORCE CONSTANT

We have this equation relating the frequency `tildenu` in wavenumbers to the force constant `k`.

`tildenu = 1/(2pic)sqrt(k/mu)`

where:

• `c` is the speed of light in `"m/s"`
• `k` is the force constant in `"kg/s"^2`
• `mu` is the reduced mass, with `mu = (m_1m_2)/(m_1+m_2)`, and `m` being the mass in `"kg"`.

`k` is introduced to you in the context of the harmonic oscillator, which is just two balls connected by a spring. `k` is larger for stronger bonds. Therefore, the higher the bond dissociation energy, the higher `k` should be.

We can see that here:
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/infrared/irspec1.htm

The `"O"-"H"` stretch is listed as `tildenu ~~ "3600 cm"^(-1)`, but the `"C"-"H"`-stretch is listed as `tildenu ~~ "3000 cm"^(-1)`.

Oxygen is smaller than carbon, so the atoms are closer together, stiffening the "spring" and strengthening the bond. Therefore, `k` is higher, and so it makes sense that `tildenu` is higher.

IONIC BONDS... DON'T STRETCH. OR BEND.

But how strong is an ionic bond? The anion is pretty much completely hogging the electrons from the cation. So is there any stretching or bending possible? Not much.

So, we wouldn't expect to see significant peaks from `"KBr"`, by itself, in the IR spectrum. We would expect too low a frequency.

That's great, because we already have to subtract the solvent background from the IR spectrum. We'd have a harder time if we also had to deal with interfering spectral peaks coming from the analytical surface.

The peaks we might actually see are due to its absorption of moisture in the air, and they should be near about `"1600 cm"^(-1)`. Other than that, not much absorption, so few interference peaks.

(Personally, if you can use NaCl plates or nujol mull, that works about as fine; KBr pellets can be expensive; the two I used were about $2000 apiece.)